Electrothermal characterization based on the third or other harmonics of an ac Joule heating current is widely deployed for the thermal analysis of solid conductors and their environment, including solid substrates and fluids. However, a unified theory that bridges heat transfer in two archetypal experimental geometries - suspended vs. substrate-supported conductor - has been missing. Here, we present and validate such a theory that explicitly accounts for finite conductor length, thermal mass, and environmental coupling through a unifiedthermal transfer function. This framework enables the prediction of voltage responses at all harmonics of the driving current (dc, 1ω, 2ω, 3ω) and the formulation of design principles for the characterization of nanoscale conductors. The conductor length l is the primary parameter controlling the frequency regimeat which the conductor's thermal mass dominates the thermal response, with the characteristic frequency ωc = α/l2, where α is the conductor's thermal diffusivity - closely related to a criterion previously reported for suspended wires free from environmental coupling. Our unified framework generalizes this result, revealingthat sufficiently weak environmental coupling is a necessary condition for ωc to govern the onset of thermal-mass-dominated response. Optimization of interfacial thermal resistance and environmental thermal impedance may further improve temperature resolution and facilitate on-substrate implementations.